Over the past few years, there has been an increasing worldwide research interest in multiple-input-multiple-output (MIMO) systems, also known as multiple-element antenna (MEA) systems, as they have been shown to have the potential for improved capacity, spectral efficiency and reliability as compared to single-antenna communication systems. MIMO technology is a breakthrough in the field of modern wireless communications, and is poised to play a significant role in the implementation of next generation's wireless products and networks.
MIMO systems consist of multiple antennas on both the transmitter and receiver ends. It is regarded as an extension to Smart Antenna systems, which consist of multiple antennas controlled by algorithms optimising their spectral and spatial efficiency.
The antenna is an extremely important component in any wireless appliance because it transmits and receives radio waves. An antenna operates as a matching device from a transmission line to free space and vice versa. An ideal antenna radiates the entire power incident from the transmission line feeding the antenna from one or more predetermined direction. Performance of the antenna dictates performance of most wireless devices and hence is a critical part of the system.
In current mobile and handheld devices, miniaturised antennas are formed by providing conductive patches and or traces on one of both sides of an insulating (or electric) substrate. The substrate may be part of a circuit board carrying other components, or part of the housing or chassis. This type of antenna may be regarded as a patch antenna, optionally with microstrip antenna features.
A number of antennas and RF circuits may be necessary to cover all the bands from 700 MHz up to 5 GHz, appropriate for applications ranging from GSM, CDMA, 3G, WiFi, Bluetooth and GPS. This not only increases the real estate required but also increases the chance of free space coupling and interference between antennas, thereby reducing the efficiency of the antenna.
Spiral antennas are well known in the art as means to provide coverage over a broad range of frequencies. The most popular configurations in the past have been Archimedean and Log spiral antennas. These have rarely made it to mobile devices in a form that has been easy to integrate either onto circuits or the chassis.
A miniaturised spiral antenna element is described in U.S. Pat. No. 6,791,497 (WO 02/29928A2), wherein the spiral antenna exists on one side of a substrate, whilst the second surface contains a planar balun and feed point which feeds the antenna element through a slot. This is said to cover a range of 800 MHz to 3 GHz. Slow wave structures are introduced as part of the spiral wind, which are said to make the device geometrically smaller for the same wavelength.
On the other hand, U.S. Pat. No. 6,295,029 B1 describes a microstrip antenna element which is formed as a rectangular spiral. This enables miniaturisation and maintains the broadband characteristics. At the end of the antenna an open stub is formed to assist in antenna matching. A ground plane for this antenna element is on the same side as the antenna, and a hole is punched in the ground to provide frequency shifting and a smaller size antenna element. However there is no discussion of the gain of the antenna, nor the impact of any neighbouring devices or other antenna elements that may affect the efficiency of the antenna element concerned.
WO 03/094293A1 discloses a miniaturised resonant slot antenna element which is fed from the back side of the substrate. Such a design is said to enhance impedance matching and reduce the physical size of the antenna, but may not improve the bandwidth of the antenna. US20060038724 describes a modified version of the same antenna, again fed from the back side of the antenna, with additional sub-slots, running clockwise and counter-clockwise, to improve the bandwidth of the antenna. The two multiple spiral slot antenna elements seen above mention the reduction of antenna size and slight improvement in the bandwidth. However it is believed that, when applied in a MIMO configuration, these implementations will not be efficient enough to provide a higher bandwidth as expected in modern portable communication systems.
U.S. Pat. No. 5,892,482 describes a circuit meant to reduce the mutual coupling between two patch antennas in an antenna for a cellular radio base station. A capacitor-inductor-capacitor network is used to couple neighbouring patch elements together, so as to inject into each one a signal in antiphase to that in its neighbour.
MIMO is expected to be a key technology to deliver the next generations of mobile devices, wherein a large number of protocols will be expected to run in parallel. This is envisaged to enable higher throughput of the devices within the limited channel bandwidth. The primary criteria of the antenna in these devices will be to reduce the co-channel interference and free-space coupling, also known as mutual coupling. There is another parameter known as envelope correlation, or cross correlation, which couples the radiation pattern of two such antennas when placed in proximity. The cross correlation can be reduced by decoupling the polarisation and placement of the antennas and by reduction of the beam width of individual antennas so they do not couple in free space, again relating to mutual coupling. However, for miniature equipment such as a mobile phone, the freedom of placement is very limited. Current design guidance is to place antenna elements at least a half wavelength apart in order to avoid grating lobes in the radiation pattern. A well-known reference in this area is R. King (1958) “Linear arrays: Currents, impedances, and fields, I”.
A particular challenge is therefore to reduce the electromagnetic interaction in the near field with neighbouring antenna elements or devices which may be amplifiers, filters, GPS receivers, camera modules, casings, battery modules, memory, sensors, and other electronic components.
WO2011102143(A1) proposes to use additional circuitry to try and adjust the mutual coupling between two antennas. The present inventors prefer to mitigate the need for such external circuitry by virtue of the antenna design itself.